This invention relates generally to transducers for measuring angular position and velocity of rotating shafts and, more particularly, to high-resolution shaft encoders that are used for precision pointing and tracking of satellite payloads.
Precision pointing and tracking of satellite payloads, including such present-day and future payloads as narrow-beam communication systems, surveillance packages and space defense systems, requires highly accurate measurements of payload angular position and velocity. One technique for measuring angular position and velocity of a satellite payload involves mounting inertial elements directly on the payload. Although this technique is frequently used for larger payloads, smaller payloads generally cannot accommodate the weight and bulk of an inertial measurement package.
Another technique for measuring angular position and velocity of a satellite payload involves mounting some type of transducer on or near the payload shaft. Conventional transducers utilized for this purpose include potentiometers, resolvers and encoders. Potentiometers and resolvers generally do not provide position and velocity measurements with sufficient accuracy for precision pointing and tracking of satellite payloads. Encoders of the contacting type, which make some physical contact with the payload shaft, are generally incorporated into ball-bearing-supported gimbal frames. Unfortunately, ball-bearing-supported gimbal frames can cause undesirable friction torques on the payload shaft. In addition, contacting type encoders rarely provide accurate velocity measurements near zero velocity because of stiction between the gimbal frames and the payload shaft.
Noncontacting type encoders, such as magnetic and optical encoders, which make no physical contact with the payload shaft, generally provide reliable angle measurements approaching one to two arc-seconds. More accurate angular measurements require encoders that are prohibitively heavy, large and costly for precision pointing and tracking of satellite payloads. In many cases, the weight of the encoder exceeds that of the payload. Furthermore, conventional noncontacting type encoders do not provide accurate velocity measurements near zero velocity because the accuracy of the velocity measurement is dependent on the velocity of the payload shaft, which is frequently very low. Accordingly, there has been a need for an improved shaft encoder that is small, lightweight and low cost and, in addition, accurately measures angular position and velocity, especially at low angular velocities. The present invention clearly fulfills this need.